Control stick force adjustment systems are used in the aircraft aviation field to provide pilots with a better "feel" and control over their aircraft by adjusting the tension of the manual control system (e.g, control stick, cyclic stick, steering, peddles, etc.,) at varying air speeds. For example, In a conventional force trim mechanism for a helicopter cyclic system, only a fixed force gradient is provided. In simple terms, for every increment of cyclic displacement, the pilot feels a proportional force. It is of course desirable that a certain amount of force is encountered in any direction a pilot moves the controller (be it left or right, forward or backward). Force on a cyclic stick provides the pilot, and ultimately the aircraft, with stability during airborne operations. The force, typically, is generated by a four bar linkage that compresses or extends a spring cartridge. Two linkage assemblies are utilized, one for lateral motion and another for longitudinal motion. By moving the spring cartridge grounding points, the position where the pilot using the cyclic stick feels zero force can be moved. Actuators called Force Trim Actuators are also used to move the spring cartridge grounding points. Because the linkages of the conventional lateral or longitudinal force trim system move in a fixed plane, these linkages are considered two-dimensional.
It should be appreciated that the force encountered in the typical helicopter operation is a substantially linear relationship. When operating an airplane, however, a pilot normally encounters a much stiffer control stick because a much higher spring force is required as the aircraft travels at higher airspeeds. Instead of moving the stick forward and backward, or the steering assembly left or right, with a normal force of one pound per inch, a pilot should encounter approximately 3 pounds per inch. Without the additional force, an aircraft flying at high speeds could undergo very erratic and dangerous aircraft movement.
Many prior controller force adjustment systems utilize electric motors to place a higher torque on the control stick, resulting in a higher tensioned feel. Although the force trim systems for some aircraft incorporate a spring tension against any force exerted by the pilot against the pilot-controlled directional gear, automated control is the predominant technology in later model aircraft. For example, in current tilt rotor aircraft applications, a variable force field actuator takes a given parameter (e.g., tilt rotor position or airspeed) and uses an electric motor to in-turn cause an increasing or decreasing force against the pilot-controlled directional system, based on inputs to the electric motor by a controller. Such a system is not only heavy but also very expensive because of the electronics in controlling the motor and the redundancy that may be required with automated systems in order to safeguard against potential system failures.
Many problems in achieving variable tension on manual controllers are unique to a tilt rotor aircraft because it functions as both an airplane and a helicopter. Because a tilt rotor aircraft operates as both, it is desirable to have the feel of the tilt rotor aircraft change as it is converted from an airplane to a helicopter, and vice a versa, during flight. The way that the "feel" and resulting handling capabilities are accomplished currently in tilt rotor aircraft systems (such as the Bell XV15 and the V22 tilt rotor aircraft), is to use the heavier, more expensive variable force field actuator systems, as described above. It would be more desirable in tilt rotor aircraft applications, and for the aircraft industry as a whole, to have access to a less complicated, lighter and more reliable variable gradient cyclic force feel system, such as disclosed in the present invention.